Photovoltaic devices including doped semiconductor films

ABSTRACT

A photovoltaic cell can include a dopant in contact with a semiconductor layer.

CLAIM FOR PRIORITY

This application claims priority under 35 U.S.C. §119(e) to ProvisionalU.S. Patent Application No. 60/985,019, filed on 2 Nov. 2007, which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to photovoltaic cells.

BACKGROUND

During the fabrication of photovoltaic devices, layers of semiconductormaterial can be applied to a substrate with one layer serving as awindow layer and a second layer serving as the absorber layer. Thewindow layer can allow the penetration of solar energy to the absorberlayer, where the optical energy is converted into electrical energy.Some photovoltaic devices can use transparent thin films that are alsoconductors of electrical charge. The conductive thin films can be atransparent conductive oxide (TCO), such as fluorine-doped tin oxide,aluminum-doped zinc oxide, or indium tin oxide. The TCO can allow lightto pass through a substrate window to the active light absorbingmaterial and also serves as an ohmic contact to transport photogeneratedcharge carriers away from the light absorbing material. A back electrodecan be formed on the back surface of a semiconductor layer. The backelectrode can include electrically conductive material, such as metallicsilver, nickel, copper, aluminum, titanium, palladium, or any practicalcombination thereof, to provide electrical connection to thesemiconductor layer. The back electrode can be a semiconductor material.Doping the semiconductor layer can improve the efficiency of aphotovoltaic device.

SUMMARY

In general, a photovoltaic cell can include a transparent conductivelayer and a first semiconductor layer in contact with the transparentconductive layer. The first semiconductor layer can include an oxidant.The transparent conductive layer can be positioned over a substrate. Inother circumstances, the transparent conductive layer can be positionedover the first semiconductor layer. The oxidant can be fluorine oroxygen, for example.

In some circumstances, a photovoltaic cell can include a layer betweenthe transparent conductive layer and the first semiconductor layer,wherein the layer contains a dopant. The dopant can include an n-typedopant. A layer containing a dopant can be in contact with a firstsemiconductor layer. A layer containing a dopant can be in contact withthe transparent conductive layer.

A photovoltaic cell can further include a second semiconductor layer incontact with a first semiconductor layer. The second semiconductor layercan include CdTe. The first semiconductor layer can include CdS. Thesubstrate can be glass.

In some circumstances, a photovoltaic cell can further include anadditional first semiconductor layer between the layer containing adopant and the transparent conductive layer. The layer containing adopant can have a thickness greater than 4 Angstroms. The layercontaining a dopant can have a thickness greater than 8 Angstroms. Thelayer containing a dopant can have a thickness greater than 12Angstroms.

In another aspect, a photovoltaic cell can include a transparentconductive layer, a first semiconductor layer in contact with thetransparent conductive layer, and a second semiconductor layer adjacentto the first semiconductor layer, wherein the first or secondsemiconductor layer includes a dopant.

In some circumstances, both the first and second semiconductor layer caninclude a dopant. The dopant in the first semiconductor layer can bedifferent from the dopant in the second semiconductor layer. Atransparent conductive layer can be positioned over a substrate. Atransparent conductive layer can be positioned over a firstsemiconductor layer. A first semiconductor layer can include CdS. Asecond semiconductor layer can include CdTe.

A dopant can be a Group III species. A dopant can be a Group I species.A dopant can be Cu, Ag, or Au. A dopant can be a Group V species. Adopant can have a concentration of greater than 0.5 ppma. A dopant canhave a concentration greater than 100 ppma. A dopant can have aconcentration greater than 500 ppma.

A method of manufacturing a photovoltaic cell can include depositing afirst semiconductor layer in contact with a transparent conductivelayer, depositing a second semiconductor layer adjacent to the firstsemiconductor layer, and introducing a dopant in the first semiconductorlayer or the second semiconductor layer.

In some circumstances, a dopant can be introduced to both the first andsecond semiconductor layer. In some circumstances, the dopant introducedto the first semiconductor layer can be different from the dopantintroduced to the second semiconductor layer.

A dopant can be introduced by diffusion. A dopant can be introduced by avapor. A dopant can be introduced as a mixed powder. A dopant can beintroduced as a solid powder. A dopant can be introduced throughout anentire semiconductor layer. A dopant can be added to a portion of alayer. A dopant can be an n-type dopant. A dopant can be a p-typedopant. A dopant can be indium and the mixed powder can be CdS andIn₂S₃.

A dopant can be introduced during heat treatment. Heat treatmenttemperature can be about 400 degrees Celsius. A dopant can be applied toa surface of a second semiconductor layer prior to heat treatment.

In another aspect, a method of manufacturing a photovoltaic cell caninclude depositing a first semiconductor layer in the presence of anoxidant and treating the first semiconductor layer with a dopant.

A first semiconductor layer can be positioned over a substrate. In othercircumstances, a first semiconductor layer can be positioned over ametal layer. The transparent conductive layer can be positioned over thefirst semiconductor layer.

An oxidant can be fluorine or oxygen. A dopant can include an n-typedopant. A dopant can include aluminum or indium. A dopant can bedistributed as a layer within a first semiconductor layer.

In another aspect, a system for generating electrical energy can includea multilayered photovoltaic cell, the photovoltaic cell including atransparent conductive layer, a first semiconductor layer in contactwith a transparent conductive layer, and a second semiconductor layerover the first semiconductor layer, wherein the first or secondsemiconductor layer includes a dopant, a first electrical connectionconnected to the transparent conductive layer, and a second electricalconnection connected to a metal layer.

In some circumstances, both the first and second semiconductor layer caninclude a dopant. In some circumstances, the dopant in the firstsemiconductor layer can be different from the dopant in the secondsemiconductor layer.

The transparent conductive layer can be positioned over a substrate. Afirst semiconductor layer can be positioned over a metal layer. Atransparent conductive layer can be positioned over the firstsemiconductor layer.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a substrate with multiple layers.

FIG. 2 is a schematic of a substrate with multiple layers.

FIG. 3 is a schematic of a substrate with multiple layers.

DETAILED DESCRIPTION

A photovoltaic cell can include a transparent conductive layer on asurface of the substrate, a first semiconductor layer, the substratesupporting the semiconductor layer, and a metal layer in contact withthe semiconductor layer. The transparent conductive layer can bepositioned over a substrate. In other circumstances, the transparentconductive layer can be positioned over the first semiconductor layer.

A photovoltaic cell can include one or more oxidants or dopants. Aphotovoltaic cell can include a first semiconductor layer including adopant or an oxidant, such as oxygen or fluorine. A photovoltaic cellcan also include a layer containing a dopant, such as an n-type dopant.A dopant can include a Group III species, such as aluminum or indium.

A layer containing a dopant can be positioned between the transparentconductive layer and the first semiconductor layer. The layer containingthe second dopant can contact the transparent conductive layer. In othercircumstances the layer containing the dopant can contact the firstsemiconductor layer. The layer containing the second dopant, such as ann-type dopant, can have a thickness of about 5-15 Angstroms, forexample.

Examples of n-type dopants include aluminum and indium. Conventionally,various defects in polycrystalline films have caused doping efficiencyof semiconductor layers to remain low. In order to improve dopingefficiency, a semiconductor layer can be treated with an oxidant such asoxygen or fluorine. After defects of the semiconductor layer have beenpassivated with an oxidant, doping efficiency can be improved. Withoutpassivation, the addition of dopants such as In or Al can have negativeeffects. Thus, doping semiconductor layers in several stages canincrease photovoltaic cell efficiency.

A dopant can be added intentionally as an extrinsic dopant. Photovoltaiccells are often processed without any intentional extrinsic dopants in afirst semiconductor layer, such as a CdS containing layer, or withoutany extrinsic dopants in a second semiconductor layer, such as CdTelayer, or both. Doping the first semiconductor layer, or secondsemiconductor layer, or both, in several stages, can reduce photovoltaiccell instability and increase efficiency.

Referring to FIG. 1, a photovoltaic cell can include semiconductorlayers that are doped in at least two stages. For example, a firstsemiconductor layer 100 can include CdS, for example. The firstsemiconductor layer, such as a CdS layer, can be treated with an oxidant101, such as oxygen or fluorine, to passivate the defects in the CdSlayer. At this point the oxidant can be introduced at an interfacebetween the transparent conductive layer and the CdS layer. Thetransparent conductive layer can include a transparent conductive oxide(TCO). Subsequently, a dopant 102, such as an n-type dopant can beintroduced. The dopant can be aluminum or indium, for example. Thedopant can be introduced to the CdS layer. The dopant can be supplied bya source such as a carrier gas or by diffusion, either from thesubstrate 130, from the TCO 120 itself, or from a surface layer 140 onthe TCO. A second semiconductor layer 150 can be deposited over a firstsemiconductor layer. A second semiconductor layer can include CdTe, forexample. A layer containing a dopant can have a thickness greater than 4Angstroms, greater than 8 Angstroms, greater than 12 Angstroms, orapproximately 15 Angstroms, for example. The structure of a photovoltaicdevice can also be reversed, for example, such that a transparentconductive layer is on a first semiconductor layer, the firstsemiconductor layer is on a second semiconductor layer, and the secondsemiconductor layer is on a metal layer.

Referring to FIG. 2, a first semiconductor layer, such as a CdS layer,can be treated with an oxidant 201, such as oxygen or fluorine, topassivate the defects in the CdS layer. Subsequently, a dopant 202, suchas aluminum or indium can be introduced to the CdS layer. The dopant cansupplied by a source such as a carrier gas or by diffusion, either fromthe substrate 230, from the TCO 220 itself, or from a surface layer onthe TCO. The dopant can form a layer between a first semiconductor layer200 a and an additional first semiconductor layer 200 b. The firstsemiconductor layer 200 a can have a thickness greater than a thicknessof the additional first semiconductor layer 200 b. For example, thethickness of the first semiconductor layer can be greater than 200Angstroms, greater than greater than 400 Angstroms, and greater than 800Angstroms, or approximately 900 Angstroms. The thickness of theadditional first semiconductor layer can be greater than 50 Angstroms,greater than 75 Angstroms, or approximately 100 Angstroms. A secondsemiconductor layer 250 can be deposited over a first semiconductorlayer. A second semiconductor layer can be treated with a second dopant.A second semiconductor layer can include CdTe, for example. A dopantlayer can have a thickness greater than 4 Angstroms, greater than 8Angstroms, greater than 12 Angstroms, or approximately 15 Angstroms, forexample.

Referring to FIG. 3, a photovoltaic device can include a firstsemiconductor layer and a second semiconductor layer. Either the firstsemiconductor layer or the second semiconductor layer can include adopant. Alternatively, both the first and second semiconductor layer caninclude a dopant. A first semiconductor layer 300 can include a dopant301. Optionally, a second semiconductor layer 350 can include a dopant302. The first semiconductor layer can be supported by a substrate 330.

Doping a semiconductor layer can be performed in several ways. Forexample, a dopant can be supplied from an incoming powder, such as amixed powder (for example, CdS+In₂S₃), a carrier gas, or a directlydoped powder such as CdS powder. The powder can be a single or multiplephase material. In other circumstances, a dopant can be supplied bydiffusion from a layer, such as a substrate layer, a transparentconductive layer, or a semiconductor layer. A dopant may be addedthroughout a semiconductor layer, or through a portion of a layer.

A dopant for a first semiconductor layer can be added by diffusion fromthe transparent conductive layer or from a surface layer on thetransparent conductive layer. Alternatively, a dopant for the firstsemiconductor layer can be added by an incoming powder or carrier gasfed into a vapor transport deposition system. A dopant for a firstsemiconductor layer can be volatilized along with the firstsemiconductor at temperatures less than 1500° C., less than 1400° C.,less than 1200° C., less than 1000° C., or less than 800° C., forexample. The volatile species can be atomic or a sulfide molecule, suchas In₂S. Indium can be an effective dopant because it has a reasonablyvapor pressure at typical CdS distributor temperatures (approximately1100° C.).

Doping a second semiconductor layer, such as CdTe, can be performed inseveral ways. For example, a dopant can be supplied along with a CdTesource powder into a vapor transport deposition (VTD) distributorsystem. The dopant for a second semiconductor layer can be introduced asa powder (i.e. a single or multiple phase material) or in a carrier gas.Alternatively, a dopant for a second semiconductor layer can beintroduced by deposition of an outer layer onto the second semiconductorlayer. For example, an outer layer may contain a dopant, which candiffuse into the CdTe layer.

P-type doping can be effective for a second semiconductor layer, such asCdTe. Group V (for example, N, P, As, Sb, or Bi) and Group I (forexample, Li, Na, K, Rb, Cs, Cu, Ag, or Au) elements can be effectivedopants for a second semiconductor layer. Phosphorous (P), arsenic (As),antimony (Sb), sodium (Na), potassium (K), rubidium (Rb) present novaporization problems when introduced into the VTD system operating attypical CdTe distributor temperatures (approximately 900-1100° C.).

A dopant for a second semiconductor layer can be introduced during heattreatment. Heat treatment can be performed with CdCl₂ flux. Group I andGroup V species such as chloride compounds, can be added to the fluxsolution that is applied to an outer surface of the second semiconductorlayer prior to heat treatment. During heat treatment, recrystallizationcan occur, thereby making doping distribution within the film possible.

Dopant incorporation can be affected by the concentration of vacancydefects. Vacancy concentration can be controlled by vapor phaseoverpressure. For example, excess Cd or Te can be applied to alterelectronic defects in the final device. Cd or Te overpressure can beachieved by introduction of excess Cd or Te in the CdTe source powder.This can be achieved by a two-phase powder blend or by anoff-stoichiometric source material. A material can be off-stoichiometricby greater than 5%, greater than 10%, or greater than 15%, such asapproximately 20%.

A dopant can be applied in any concentration, for example, greater than0.5 ppma, greater than 50 ppma, greater than 500 ppma, or between 1-1000ppma, for example. Certain gas source dopants, such as those containingnitrogen, for example, may decompose too readily in a heated CdTedistributor, and thus, can be introduced in a secondary, lowertemperature distribution system. Dopants can be added throughout anentire semiconductor layer, or into only a portion of the layer by usingmulti-layer VTD configuration, for example.

A CdS layer can be treated with a dopant such as indium in several ways.In one example, the incorporation of indium in a CdS layer via In₂S₃ anda source powder has resulted in improved luminescence. In anotherexample, a CdS layer was doped directly with indium, and extra sulfurlead to an increase in indium incorporation.

A CdTe layer can also be treated with a dopant such as sodium in severalways. In one example, a CdTe layer was doped with sodium using Na₂Spowder. The addition of Na resulted in changes in recrystallizationduring subsequent chloride heat treatment, suggesting that Na dopingmade the CdCl₂ recrystallization flux more active. Similar results werefound with RbCl doping.

A CdTe layer can also be doped with P, As, Sb, Na, Rb, or Cu. In yetanother example, a CdTe layer doped with Cu was shown to exhibitimproved electrical signals. The addition of extra Te to the CdTe powderresulted in improved ability to dope the films. In addition, Cd-richconditions resulted in a change in film grain structure suggesting theimproved ability to dope the films.

A common photovoltaic cell can have multiple layers. The multiple layerscan include a bottom layer that is a transparent conductive layer, acapping layer, a window layer, an absorber layer and a top layer. Eachlayer can be deposited at a different deposition station of amanufacturing line with a separate deposition gas supply and avacuum-sealed deposition chamber at each station as required. Thesubstrate can be transferred from deposition station to depositionstation via a rolling conveyor until all of the desired layers aredeposited. Additional layers can be added using other techniques such assputtering. Electrical conductors can be connected to the top and thebottom layers respectively to collect the electrical energy producedwhen solar energy is incident onto the absorber layer. A top substratelayer can be placed on top of the top layer to form a sandwich andcomplete the photovoltaic cell.

The bottom layer can be a transparent conductive layer, and can be, forexample, a transparent conductive oxide such as tin oxide or tin oxidedoped with fluorine. Deposition of a semiconductor layer at hightemperature directly on the transparent conductive oxide layer canresult in reactions that negatively impact of the performance andstability of the photovoltaic device. Deposition of a capping layer ofmaterial with a high chemical stability (such as silicon dioxide,dialuminum trioxide, titanium dioxide, diboron trioxide and othersimilar entities) can significantly reduce the impact of these reactionson device performance and stability. The thickness of the capping layershould be minimized because of the high resistivity of the materialused. Otherwise a resistive block counter to the desired current flowmay occur. A capping layer can reduce the surface roughness of thetransparent conductive oxide layer by filling in irregularities in thesurface, which can aid in deposition of the window layer and can allowthe window layer to have a thinner cross-section. The reduced surfaceroughness can help improve the uniformity of the window layer. Otheradvantages of including the capping layer in photovoltaic cells caninclude improving optical clarity, improving consistency in band gap,providing better field strength at the junction and providing betterdevice efficiency as measured by open circuit voltage. Capping layersare described, for example, in U.S. Patent Publication 20050257824,which is incorporated by reference in its entirety.

The window layer and the absorbing layer can include, for example, abinary semiconductor such as group II-VI, III-V or IV semiconductor,such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO,MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, MnO, MnS, MnTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,or mixtures thereof. An example of a window layer and absorbing layer isa layer of CdS coated by a layer of CdTe. A top layer can cover thesemiconductor layers. The top layer can include a metal such as, forexample, aluminum, molybdenum, chromium, cobalt, nickel, titanium,tungsten, or alloys thereof. The top layer can also include metal oxidesor metal nitrides or alloys thereof.

Deposition of semiconductor layers in the manufacture of photovoltaicdevices is described, for example, in U.S. Pat. Nos. 5,248,349,5,372,646, 5,470,397, 5,536,333, 5,945,163, 6,037,241, and 6,444,043,each of which is incorporated by reference in its entirety. Thedeposition can involve transport of vapor from a source to a substrate,or sublimation of a solid in a closed system. An apparatus formanufacturing photovoltaic cells can include a conveyor, for example aroll conveyor with rollers. Other types of conveyors are possible. Theconveyor transports substrate into a series of one or more depositionstations for depositing layers of material on the exposed surface of thesubstrate. Conveyors are described in provisional U.S. application Ser.No. 11/692,667, which is hereby incorporated by reference.

The deposition chamber can be heated to reach a processing temperatureof not less than about 450° C. and not more than about 700° C., forexample the temperature can range from 450-550° C., 550-650° C.,570-600° C., 600-640° C. or any other range greater than 450° C. andless than about 700° C. The deposition chamber includes a depositiondistributor connected to a deposition vapor supply. The distributor canbe connected to multiple vapor supplies for deposition of various layersor the substrate can be moved through multiple and various depositionstations with its own vapor distributor and supply. The distributor canbe in the form of a spray nozzle with varying nozzle geometries tofacilitate uniform distribution of the vapor supply.

The bottom layer of a photovoltaic cell can be a transparent conductivelayer. A thin capping layer can be on top of and at least covering thetransparent conductive layer in part. The next layer deposited is thefirst semiconductor layer, which can serve as a window layer and can bethinner based on the use of a transparent conductive layer and thecapping layer. The next layer deposited is the second semiconductorlayer, which serves as the absorber layer. Other layers, such as layersincluding dopants, can be deposited or otherwise placed on the substratethroughout the manufacturing process as needed.

The transparent conductive layer can be a transparent conductive oxide,such as a metallic oxide like tin oxide, which can be doped with, forexample, fluorine. This layer can be deposited between the front contactand the first semiconductor layer, and can have a resistivitysufficiently high to reduce the effects of pinholes in the firstsemiconductor layer. Pinholes in the first semiconductor layer canresult in shunt formation between the second semiconductor layer and thefirst contact resulting in a drain on the local field surrounding thepinhole. A small increase in the resistance of this pathway candramatically reduce the area affected by the shunt.

A capping layer can be provided to supply this increase in resistance.The capping layer can be a very thin layer of a material with highchemical stability. The capping layer can have higher transparency thana comparable thickness of semiconductor material having the samethickness. Examples of materials that are suitable for use as a cappinglayer include silicon dioxide, dialuminum trioxide, titanium dioxide,diboron trioxide and other similar entities. Capping layer can alsoserve to isolate the transparent conductive layer electrically andchemically from the first semiconductor layer preventing reactions thatoccur at high temperature that can negatively impact performance andstability. The capping layer can also provide a conductive surface thatcan be more suitable for accepting deposition of the first semiconductorlayer. For example, the capping layer can provide a surface withdecreased surface roughness.

The first semiconductor layer can serve as a window layer for the secondsemiconductor layer. The first semiconductor layer can be thinner thanthe second semiconductor layer. By being thinner, the firstsemiconductor layer can allow greater penetration of the shorterwavelengths of the incident light to the second semiconductor layer.

The first semiconductor layer can be a group II-VI, III-V or IVsemiconductor, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, MnO, MnS, MnTe,AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN,TlP, TlAs, TlSb, or mixtures or alloys thereof. It can be a binarysemiconductor, for example it can be CdS. The second semiconductor layercan be deposited onto the first semiconductor layer. The secondsemiconductor can serve as an absorber layer for the incident light whenthe first semiconductor layer is serving as a window layer. Similar tothe first semiconductor layer, the second semiconductor layer can alsobe a group II-VI, III-V or IV semiconductor, such as, for example, ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS,HgSe, HgTe, MnO, MnS, MnTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, or mixtures thereof.

The second semiconductor layer can be deposited onto a firstsemiconductor layer. A capping layer can serve to isolate a transparentconductive layer electrically and chemically from the firstsemiconductor layer preventing reactions that occur at high temperaturethat can negatively impact performance and stability. The transparentconductive layer can be deposited over a substrate.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. For example, the semiconductorlayers can include a variety of other materials, as can the materialsused for the buffer layer and the capping layer. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A photovoltaic cell comprising: a transparent conductive layer; a first semiconductor window layer over the transparent conductive layer; a dopant layer comprising a first dopant over the first semiconductor window layer; a second semiconductor window layer over the first semiconductor window layer, wherein the dopant layer is between the first semiconductor window layer and the second semiconductor window layer, and wherein the second semiconductor window layer includes an oxidant and the first dopant; and a semiconductor absorber layer over the second semiconductor window layer; wherein the first and second semiconductor window layers comprise CdS; and wherein the dopant layer containing the first dopant has a thickness between 4 and 15 Angstroms.
 2. The photovoltaic cell of claim 1, wherein the transparent conductive layer is positioned over a substrate.
 3. The photovoltaic cell of claim 1, wherein the oxidant comprises fluorine or oxygen.
 4. The photovoltaic cell of claim 1, wherein the first dopant includes an n-type dopant.
 5. The photovoltaic cell of claim 1, wherein the dopant layer comprising the first dopant is in direct contact with the first semiconductor window layer.
 6. The photovoltaic cell of claim 1, wherein the semiconductor absorber layer comprises CdTe.
 7. The photovoltaic cell of claim 1, further comprising a glass substrate, wherein the transparent conductive layer is over the glass substrate.
 8. The photovoltaic cell of claim 1, wherein the second semiconductor window layer includes the first dopant at a concentration greater than about 100 ppma.
 9. The photovoltaic cell of claim 1, wherein the semiconductor absorber layer includes a second dopant, wherein the first dopant is different from the second dopant.
 10. The photovoltaic cell of claim 9 wherein the first dopant comprises a Group III species.
 11. A photovoltaic cell comprising: a transparent conductive layer; a first semiconductor window layer over the transparent conductive layer; a layer comprising a first dopant over the first semiconductor window layer; a second semiconductor window layer over the first semiconductor window layer, wherein the layer comprising the first dopant is between the first semiconductor window layer and the second semiconductor window layer, and wherein the second semiconductor window layer includes an oxidant and the first dopant; and a semiconductor absorber layer over and in direct contact with the second semiconductor window layer, wherein the semiconductor absorber layer comprises a second dopant; wherein the first and second semiconductor window layers comprise CdS; and wherein the layer comprising the first dopant has a thickness between 4 and 15 Angstroms.
 12. The photovoltaic cell of claim 1, further comprising a back metal electrode positioned over the semiconductor absorber layer.
 13. The photovoltaic cell of claim 3, wherein the oxidant comprises fluorine.
 14. The photovoltaic cell of claim 11, further comprising a back metal electrode positioned over the semiconductor absorber layer.
 15. The photovoltaic cell of claim 11, wherein the oxidant comprises fluorine.
 16. A photovoltaic cell comprising: a transparent conductive layer; a first semiconductor window layer over the transparent conductive layer; a dopant layer comprising a first dopant over and in direct contact with the first semiconductor window layer; a second semiconductor window layer over and in direct contact with the dopant layer; wherein the second semiconductor window layer is thicker than the first semiconductor window layer, and wherein the second semiconductor layer includes an oxidant and the first dopant; and a semiconductor absorber layer over the second semiconductor window layer, wherein the semiconductor absorber layer further comprises a second dopant; wherein the first and second semiconductor window layers comprise CdS; and wherein the dopant layer comprising the first dopant has a thickness between 4 and 15 Angstroms.
 17. The photovoltaic cell of claim 11, wherein the first dopant in the second semiconductor window layer is at a concentration greater than about 100 ppma.
 18. The photovoltaic cell of claim 16, wherein the oxidant comprises fluorine or oxygen.
 19. The photovoltaic cell of claim 16, wherein the first dopant has a concentration greater than 100 ppma in the second semiconductor window layer.
 20. The photovoltaic cell of claim 1, further comprising a capping layer between the transparent conductive layer and the first semiconductor window layer.
 21. The photovoltaic cell of claim 11, further comprising a capping layer between the transparent conductive layer and the first semiconductor window layer.
 22. The photovoltaic cell of claim 16, further comprising a capping layer between the transparent conductive layer and the first semiconductor window layer. 